The present invention relates to a process for manufacturing hollow bodies by shaping a preform, which is in the form of a hollow longitudinal body or sheets of aluminum or its alloys, by means of a high internal pressure process or superplastic forming in a tool, the use of the hollow body and the preform for performing the process.
It is known to shape-form sheets of metal by superplastic forming. In order to counter different degrees of elongation of the material during forming, it has been proposed in Swiss Patent 673 603 to cool the sheet locally during the forming operation.
In U.S. Pat. No. 4,269,053 a process for superplastically forming parts is described in which a preform is pressed against a shape-endowing part. The shape-endowing part exhibits various coefficients of friction. Accordingly, the material of the preform can expand more easily and a more uniform distribution of thickness is obtained in the final part.
A prerequisite for these processes is for example that the tools have to be equipped with inlets for cooling medium or, a third material has to be introduced into the tool as an intermediate layer.
In the process of superplastic forming, the sheet to be formed is clamped in a chamber and heated to the forming temperature. For aluminum alloys this lies between approx. 450.degree. and 550.degree. C. The forming of the sheet takes place by means of compressed air in a counterforming tool in the chamber. The unavoidably different local elongation of the material that arises during shape-forming leads to smaller cross-sectional thicknesses in the regions that are elongated to a greater degree. This localized necking of the material has the result of preferred concentration of the further elongation of material in these regions.
In the case of high internal pressure forming a preform is introduced into a divided tool which is then closed and two compression side pieces with pushing rods advanced. After the tubular shaped piece has been filled with the pressure medium and the signal given to the high pressure source, the actual forming process is started. The high internal pressure which the pressure medium applies to the preform, in some cases in combination with the superimposed axial pressure, causes the preform to take on the precisely the contour of the tool. It is also possible to feed limited amounts of material by applying axial compression during the spreading out stage.